In-Game Lag Spikes (Skipping; Video & Audio)

I've seen a few help topics like this one but mine only relates to In-Game lag not lagging offline or just on windows itself.
When I play Call of Duty 2 online every few seconds there's a lag spike, but my ping is fine. I guess it's more of a skip, the sound skips as well it happens about every 3 seconds and in a pattern not at random. Which makes it really annoying and hard to ignore when playing. This also happened a few times when playing America's Army but not as much. Sometimes the game is fine though last night i was playing fine but I went on a few hours ago and it wouldn't go away.
I also have Medal of Honor and it doesn't happen at all with that game.
It's not my system my specs are:
-256mb PCI-E graphics
-1.5 GB ram
-320 GB of HDD
-ATI Mobo
-Windows XP Pro sp2

Again this only happened recently, it happened with BF2 a while ago but I uninstalled that ages ago. And I haven't installed any new software since this started occuring again. Just ran a virus scan and it comes back clean.
Thanks for your help COD2 is a great game and hopefully ill be able to get back into the action.

Okay I did a virus scan and nothing came up, i defragged, "cleaned up" the disk, and reinstalled itunes.
(Music works fine in windows media player but not in itunes)
But now i'm still getting the lag in the game, itunes skips, and my computer is really slow...(keep in mind i have 1.5gb of ram and am only using 30% of my internal HDD.
My friend told me it could missing or corrupt system files, is there something that i can download that will check if i have all of the neccesary system files and if any are corrupt? I dont have the xp pro installation cd, it came preinstalled but i do have a xp home installation cd.

yea i uninstalled and updated it and it still skips, so for now im using windows media player to listen to songs on my computer and just itunes to update my ipod. I played a movie before in windows media and it was fine but in divx it was a little screwy

its not skipping as much but it still is, the only one i could roll back was the audio driver. The video said theres no backup files. And i cant roll back the audio twice, the second time i tried it, the window crashed and it said those "report this" message about kernel and drwatson